Carbon fiber has previously been produced from polyethylene fiber by liquid immersion sulfonation of the polyethylene fiber (e.g., by treatment with chlorosulfonic or sulfuric acid), followed by pyrolysis. The sulfonation step makes the polyethylene fiber thermally infusible, and thus, carbonizable at the high temperatures employed for carbonization.
However, the liquid immersion sulfonation process, as conventionally practiced, has at least the significant drawback of being either very slow with respect to the degree of sulfonation provided to the polyethylene fiber, or very aggressive such that the reaction is uncontrollable before it achieves equilibrium or complete sulfonation (i.e., a saturated level of sulfonation) of the precursor fiber. Depending on the type of precursor, complete sulfonation preferably occurs through the core of the fiber and maintains a gradient in the degree of functionalization across the filament radius.
Since carbon yield and carbon fiber properties (e.g., strength, brittleness, and fracture toughness) are dependent on the degree of sulfonation, there would be an advantage in adjusting the degree of sulfonation of the precursor in order to accordingly adjust the properties of the carbon fiber. However, the methods known in the art are generally not amenable for such careful adjustment in the degree of sulfonation because the aim has heretofore been to achieve complete sulfonation of the precursor to produce solid carbon fiber.